In order to enhance the performance of plastic tubular product and give it new functions, it often requires embellishing and processing of the outer surface of plastic tubular product, e.g. applying certain material coatings onto plastic tubular product so as to enhance its strength, toughness, elasticity modulus, oxygen resistance, anti-static property, fire resistance, etc.
In the extrusion coating apparatus of the prior art, when a modified polymer layer is coated onto plastic tubular product, the polymer melt flows in a direction parallel to the axis of the tubular product, and because the extrusion coating process is carried out in the state of extensional flow, the macromolecular chains of the polymer tend to form certain alignment along the axial direction of the tubular product, so that the axial strength of the tubular product is greater than its circumferential strength. According to stress analysis of pressure-bearing plastic tubular product, during its operational process, the stress generated circumferentially in the tube wall is more than twice the stress generated parallel to the tube axis in the tube wall, therefore, for ordinary pressure-bearing plastic tubular product, the circumferential direction with the greatest stress just happens to be the weakest direction of tube strength. This kind of anisotropy of strength is very unreasonable for tubular product used for transporting pressurized fluid.
Chinese patent literature CN101332667A disclosed an extrusion machine head for long glass fiber reinforced plastic tubular products, particularly an extrusion machine head for round plastic tubular products which is installed on a plastic extruder and has the characteristic of online spiral compounding, this apparatus has a device for glass fiber import and mixing dispersion of fiber in the melt, as well as a device for mandrel rotation, the mixing dispersion device of this machine head achieves that glass fiber with certain length which has been processed by coupling agent is imported into plastic melt which has already been plasticized, and that the glass fiber is evenly mixed into the plastic melt so as to form fiber-plastic mixed melt, and the mandrel rotation device of this machine head achieves by the rotation of the mandrel that the melt between the mandrel and the die cavity wall is changed from flowing in the axial direction of the mandrel to flowing in a spiral direction around the mandrel, until it solidifies. The polymer molecule and long glass fiber within the plastic tubular product extruded by the above-mentioned apparatus are aligned in the spiral direction around the axis, so that the circumferential strength of the tubular product is greatly enhanced.
However, the circumferential alignment of long glass fiber within the above-mentioned tubular product is achieved by the rotation of the mandrel in the die cavity, thus needs external driving power source and corresponding transmission means (such as a mandrel speed regulation chain wheel, a chain wheel holder for transmission of mandrel rotation, a transmission chain of mandrel rotation, a rotatable mandrel and a transmission driven chain wheel), which is both energy consuming and structurally complicated; moreover, the above-mentioned apparatus has an inevitable defect that, as the melt is rotated with the mandrel and is gradually pushed out of the die cavity so as to be cooled and molded, the rotation stress existing within the tube blank will maintain until the tubular product is cut apart, therefore, besides the necessary rectilinear motion, an unwanted rotation motion is added to the tubular product, which is not allowed in the extrusion molding process of all thermoplastic plastic tubular products; moreover, the melt temperature in the extrusion process of most plastic is about 200° C., some up to 400° C., the melt pressure in the extrusion process of plastic is up to several megapascal or several tens of megapascal, and the melt pressure of polymer added with glass fiber is even higher, which leads to difficulty of sealing between the mandrel and the bearing pedestal, therefore, it easily causes leakage of melt, negatively affect normal functioning of the equipment, adds to the labor intensity of the operator, and sometimes leads to damage of the equipment.